An oxide superconductor is a high temperature superconductor having a critical temperature (Tc) exceeding 23K! (Tc) of a conventional superconductor Nb.sub.3 Ge. Particularly, a superconductor (R123 superconductor) of the formula RBa.sub.2 Cu.sub.3 O.sub.7-c wherein R is a rare earth element including Y and lanthanoid, and 0.ltoreq.c.ltoreq.1 has a Tc reaching as high as 90K!.
An R123 superconductor is prepared by, for example, a melting method. Taking Y123 (YBa.sub.2 Cu.sub.3 O.sub.7-c) for example, a starting material is heated to once give a semi-melt of Y.sub.2 BaCuO.sub.5 (Y211) phase and a liquid phase, and cooled to a crystallization temperature to give Y123 crystals.
Of the R123 superconductors formed by the melting method, Y123 has been known to have a higher critical current density (Jc).
Conventionally, for the preparation of an R123 (particularly Y123) superconductor having a high Jc by the above-mentioned melting method, the Jc of Y123 is increased by microdispersion of a non-superconductive phase Y.sub.2 BaCuO.sub.5 (Y211) in the superconductive phase Y123 during the crystal growth thereof. In this case, the Y211 phase is considered to act as a pinning center. Then, the obtained crystals are heat treated (introduction of oxygen) to cause transition of the crystal structure of Y123 from tetragonal one to rhombic one to give a superconductor.
According to the above-mentioned melting method, charging composition (ratio of constitutive elements of the starting material), kind and amount of additives such as Pt, crystal growth temperature and the like are adjusted to control the amount and size of the Y211 phase to be dispersed during the crystal growth of the Y123.
Of the R123 superconductors prepared by the conventional melting method, Y123 alone substantially has a high Tc and a high Jc.
Experimental reports have documented in recent years that an R123 wherein R is an element other than Y was prepared by the melting method while controlling oxygen partial pressure. Yet, the Jc peak of the obtained superconductor was in about 1T!, and a superconductor having a Jc peak in a higher magnetic field has not been produced.
The one obtained by microdispersion of a non-superconductive phase in a superconductive phase during the crystal growth like the above-mentioned mentioned Y123 has a pinning center like Y211 phase, which pinning center having a size of a micron order, so that it cannot increase Jc in the local magnetic field but monotonously reduces Jc with increasing magnetic fields.
Thus, an object of the present invention is to provide a superconductor and a precursor therefor which cannot be obtained by a conventional production method, said superconductor having a high Jc or having different properties with respect to Jc, by the use of the same conventional materials.
Another object of the present invention is to provide use of the superconductor of the present invention.
A still another object of the present invention is to provide the production method of the above-mentioned superconductor and a precursor therefor.